1. Field of the Invention
This invention relates to a method for producing a region of high energy capable of sustaining reactions that require high energy such as fusion and the production of powders characterized by very fine particle size and selected composition and in particular to a method involving high voltage discharge generated on an electrode surface in a dielectric liquid.
2. Prior Art
Efforts to generate regions of high energy on the surfaces of electrodes to support reactions requiring high energy have been motivated by interest for many applications--such as the generation of powders, fusion, etc. Motivation for generating powders have included applications in catalysis, magnetic recording media, jet fuel, metallurgy, ceramics, printed circuits, etc.
The prior art technology of generating powders of materials, e.g., metals and metal compounds, includes precipitation from solution, decomposition of gases, gas-water atomization, evaporation techniques, spark erosion, etc. An important objective in all of this activity is to produce particles characterized by small size.
Spark erosion is a technique whereby an electric current is passed between surfaces of material to be eroded resulting in a spark discharge that erodes the surfaces. The electrodes may be immersed in a dielectric such as distilled water or a hydrocarbon which may or may not react with the products of the discharge to produce a compound whose composition is different from that of the reacting surfaces.
Two general approaches to the technology of spark discharge have been disclosed.
One approach (see U.S. Pat. No. 3,355,279) discloses an apparatus comprising two electrodes between which are many chunks of material having the same composition as the electrodes. Electrodes and chunks are immersed in a liquid dielectric such as distilled water or hydrocarbon. A voltage is impressed between the electrodes while the electrodes and chunks are agitated. The jostling causes the chunks and electrodes to make and break contact with one another so as to generate sparks. Generation of sparks cause erosion of the chunks and electrodes. to form a powder. By selecting the appropriate composition of chunks, electrodes and dielectric liquid, particles can be formed that are not only very small, but which also have compositions that depend on sparking conditions and composition of the dielectric and material being eroded.
Influence of sparking conditions and electrode material on particle size, composition and crystal structure is discussed in "Spark Erosion" by A. Berkowitz et al published in the Journal of Material Research 2 (2) Mar. 1987. For example, U.S. Pat. 3,355,279 discloses formation of aluminum oxide powders by spark erosion of aluminum electrodes in water.
Another approach involves an apparatus in which the surfaces of two electrodes are immersed in a dielectric and maintained in very close proximity to one another by servo means so as to sustain a continuous arc and thereby erode the surfaces to produce the fine powder. See "RESA--A wholly New Process for Fine Oxide Powder Preparation" published in the Journal for Materials Research, 3 (6) Nov., 1988.
Additional references include U.S. Pat. No. 4,416,751 which discloses production of a ferro fluid by spark discharge; U.S. Pat. No. 4,759,905 which discloses a "Method for Fabrication of Low Cost Finely Divided Si-Ge."
Powder technology has been an essential element for developing media for magnetic recording. For many years, recording films have been made by mixing ferrite powders with a precured resin, spreading the coating onto the appropriate substrate (disk or tape), heat curing the film, polishing the film to a required thickness. The powders are produced by chemical precipitation which generally comprises mixing an aquaeous solution of an iron salt such as ferric chloride with a base such as sodium hydroxide to precipitate finely divided ferrous hydroxide. The powder is then washed to remove the sodium salt. The product at this point is a hydrate of ferrous or ferric oxide which is nonmagnetic. To convert the powder to the magnetic phase, the powder is calcined in a reducing atmosphere at a temperature generally above 500.degree. C. to drive off the waters of hydration and convert the powder to the magnetic phase. The calcining step presents the problem that the particles tend to coagulate during the heat-dehydration process.
One of the important advances in the art of generating magnetic powders of iron oxide has been the discovery that the incorporation of controlled amounts of various metal ions into the ferrite lattice results in particles that have greatly improved properties for magnetic recording. A principle result has been an increase in coercive force, the magnetic field required to remagnetize the particle. Incorporation of various metal constituents,(e.g., Co) has been accomplished by adding the appropriate salt, e.g., CoC1, to the solution for coprecipitation of iron and cobalt to produce a Cobalt containing iron oxide.
The addition of Barium has been especially effective for increasing coercive force. For example, Ferrites containing no metal ions other than iron typically have a coercive force of 300 oe. Addition of Barium in an amount equal to 1:12 can increase coercive force to above 1000 oe.
Another important property for recording powder particles is particle shape and size. In particular, coercive force is increased by reducing particle size. Present production methods produce particles in the range of 100 to 300 nanometers having generally acicular shape. Magnetic powders characterized by a range of size and acicular shape present a range of coercive forces rather than a discrete value so that the bit information storage density capability of a coating made with such particles is limited.
In view of the above, a process for producing Barium Ferrite has been developed which involves growing the particles in a flux then washing away the flux.
The techniques of spark erosion to produce powders has a number of advantages over competing techniques such as precipitation methods. One advantage is that the process is much simpler since the problem of removing products of reaction such as sodium chloride is avoided. Furthermore, in some applications, the necessity of having to calcine is avoided. Production of powders having particle size smaller than the other techniques is often achieved.
However, in many instances, a specific composition of powders which are alloys or combinations of metal oxides is difficult to achieve using spark erosion. The problem results from preferential volatilization of a particular species of atom which results in loss of one constituent from an electrode at a faster rate than loss of a second constituent. The result is an ever changing composition of both the composition of the electrode and powder formed thereby.
For example, during the course of reduction to practice of this invention, Barium Ferrite powder was generated by spark erosion using electrodes and chunks of barium ferrite formed by sintering compacts of coarse particles whose initial composition was 12 to one atomic ratio of iron to barium. Samples of powder were made using a range of discharge voltages from 150 to 500 volts discharging capacitors ranging from one to ten microfarads. Chemical analysis showed that the atomic ratio of Iron to Barium ranged from four to one up to nine to one indicating that the barium volatilized from the original ferrite at a greater rate than the iron.
Another problem with prior art methods of spark erosion is the wide range of particle size that is generated. In the work discussed in the foregoing paragraph, particle size ranged from micron size down to a few hundred Angstroms so that expensive separation techniques are required to remove the larger particles from the powder charge in order to obtain a powder that is satisfactory for magnetic recording.